1. Field of the Invention
The present invention concerns a radio-frequency volume coil for (MR) tomography apparatus.
2. Description of the Prior Art
A volume coil for MR tomography systems is known from DE 195 09 731 A1. Such volume coils are used in the gradient system of an MR scanner as RF transmission and/or reception coils. Radio-frequency fields for the excitation of the spins are generated and the generated signal is received. If the volume coil operates exclusively or also as a reception coil, its size (i.e. the size of the volume that the volume coil encloses) in relation to the size of the measurement subject plays a determining role for the resulting signal/noise ratio. An optimal signal/noise ratio is achieved when the volume coil is situated optimally close to the contour of the measurement subject.
For this reason it is known in the prior art to implement examinations on the head of a patient with a special head volume coil arrangement. The coil diameter is correspondingly smaller than the diameter of the volume coil with which measurements are conducted on the torso of the patient. In this technique it furthermore remains disadvantageous that these volume coils each has a fixed size. The dimensions of such a coil always are only a compromise since the subjects to be examined (here, for example, the heads of patients) have different sizes.
In head examinations three rough classifications can be made:                (1) the subject has a small head,        (2) the subject has a large head, and        (3) the subject has a head fitted with a stereotaxic frame.        
In the latter case the internal volume of the coil must be so large that the stereotaxic frame still has space, which leads to the situation that measurements conducted in such a volume coil system on small or even large heads lead to signals with a reduced signal/noise ratio. In reverse, if one makes the coil volume so small that good results are achieved given small or large heads, these volume coil arrangements are not suitable for measurements with a stereotaxic frame.
DE 195 09 371 A1 describes a modifiable antenna for a magnetic resonance apparatus with an antenna conductor that can be adapted to examination areas of varying size. This ensues in that a region of the antenna conductor is divided into individual conductor pieces whose ends can be connected with one another via loop contacts. A conductor piece is surrounded by a housing which is fashioned as a sheath or jacket. The conductor piece has loop tracks arranged on an inside of the sheath and belonging to the loop contact, such that a loop accepter on a further conductor piece can be inserted into the sheath and can be brought into contact with the loop contact at various positions.
In this antenna arrangement, the loop contacts are located at a point that is not visible, such that neither the electrical connection nor the mechanical contact can be checked.